DJS-chain

This code illustrates a chain of linked objects from browsers to a single database via two servers using HTTP and WebSocket.

Main Features

This code is part of the Diffuse Javascript MOOC. It illustrates a chain of objects starting in the browser, linked to server objects and backed up in an SQL database. When an object is modified in a browser, an HTTP REST API call propagates the modification towards the corresponding server object which is persisted in an SQL database, the modification is then pushed towards the various clients of the server via WebSocket so, at the end, all browsers should eventually share the same state.

browserObject.setProperty(propName, newValue)
   ==> HTTP REST request to the server:
       ===> convert HTTP request into javascript code:
            serverObject.setProperty(propName, newValue)
            ===> convert modification into SQL: 
                 update table set propName = newValue
       <==  Broadcast updates to all browser clients via JSON and WebSocket
       {update: propName, value: newValue}

The code depends only on the sqlite3 and ws modules, it does not use elaborate framework for Web, ORM, REST, etc. but just plain standard Node.js modules: http, url and fs. This keeps code minimal, simple and disembarrassed of useless features.

Files required for servers

dbobject is a tiny ORM (Object Relational Mapping) converting rows of SQL tables into Javascript objects. Conversely, modifying the Javascript object runs appropriate SQL commands to update the database.

webapi is a tiny HTTP server that offers a REST API mapping HTTP requests into access to database objects. HTTP methods such as GET, PUT, POST and DELETE are supported.

wsapi is a tiny WebSocket server that pushes modifications of database objects to all connected clients. webapi could also have been implemented with WebSocket but this separation illustrates the different aspects of the two protocols.

Files required for clients

browserobj is a tiny library that offers objects linked to remote objects hosted on some server and accessed via an HTTP REST API. A modification to such an object is translated into an HTTP PUT request. Concurrently, the library fires a WebSocket client receiving the updates to apply.

Installation

The simplest way is to install the npm module djs-chain with

npm install djs-chain
cd node_modules/djs-chain/

If you prefer to clone the repository, then you must regenerate the webapp with the following commands:

git clone https://github.com/ChristianQueinnec/DJS-chain.git
cd DJS-chain
gulp

Demo

Start a server (by default on port 18080) with:

npm run start

If you prefer another port, you may specify it explicitly with:

export PORT=18080
cd Site
node ../src/chain.js

Note that websocket will use the next port (by default 18081).

Once the server is started, fire a browser on http://127.0.0.1:18080/. The served page shows two independent frames, you may modify objects in one frame and observe the modifications arriving in the other frame (via the server). A small documentation appears on the served page.

Miscellaneous

OpenAPI

An OpenAPI description of the HTTP REST API is available in Swagger/swagger.json. You may visualize it on the Swagger site

Heroku

Does Heroku supports two simultaneous open ports (http and ws) ?